In the thesis a new cucumber disease in Ukraine, caused by Acremonium cucurbitacearum, is described, the mechanism of its phytopathogenic effect is researched, a screening is made and Trichoderma viride 017 fungus, which is characterized by a high antagonistic activity against A. cucurbitacearum and other agents causing root diseases of plants, is selected. Acremonium sp. 502 fungus was referred to the species of A. cucurbitacearum Alfaro-Garcia, W. Gams et J. Garcia-Jimenez according to its morphological and cultural, physiological and biochemical, and molecular and genetic characteristics. Pathogenicity of A. cucurbitacearum 502 for cucumber plants was confirmed during the pot experiment. It was established that Acremonium sp. 502 is localized exclusively in the root system, the root collar and the hypocotyl. The symptoms of acremiosis affecting roots were observed already after 14 days of growing plants in the soil infected with A. cucurbitacearum 502. The ability of A. cucurbitacearum 502 to form intracellular mycelium in epidermal and parenchymal tissues of the root is shown. Main tissues are not colonized, which indicates at the impossibility of clogging the vessels of plants with mycelium and points out to the biochemical effect of the fungus causing the process of pathogenesis on plants.
The above-ground symptoms are manifested in the yellowing of leaf edges with their subsequent browning. It was established that A. cucurbitacearum 502 culture fluid does not exhibit phytotoxic activity and does not alter the proliferative activity of plant cells, but violates the cell cycle dynamics. We showed that the fungus is capable of an active synthesis of cellulase enzymes, due to which the cell wall of the plant degrades and allows the fungus to penetrate into the cells and create intracellular mycelium. The highest cellulolytic activity was detected after 6 weeks of fungi cultivation with a pH indicator of 8.5. In the process, the total cellulolytic activity was 1.95 U/ml, exogluconase activity – 3.23 U/ml, endogluconase activity – 2.85 U/ml and β-glucosidase activity – 2.39 U/ml. The ability of A. cucurbitacearum 502 to biosynthesize ethylene was revealed, and the highest level of it was registered after 5 weeks of cultivation (111.78 ± 13.27 nmol×h-1×g-1), which coincided with the terms of the highest level of cellulases synthesis. As it is known that ethylene influences the cellulases activity and accelerates leaves aging, it can be assumed that its synthesis causes the process of pathogenesis. The next stage was a search for the active microorganism-antagonist of the A. cucurbitacearum 502 fungus, which was carried out among the bacteria of the Bacillus genus and the fungi of the Chaetomium and Trichoderma genera. It was established that the strain of Trichoderma sp. 017 fungus was characterized by the fastest growth and manifested hyperparasitism to A. cucurbitacearum 502 on the 5th day of cultivation of the former. A study of morphological and culture features allowed us to refer Triсhoderma sp. 017 to T. viride 017 species. A study of T. viride 017 strain pathogenicity to warm-blooded animals showed the absence of fungal invasion in the internal organs of animals, which led to a conclusion that it is nonpathogenic. Three-year field experiments proved that the introduction of A. cucurbitacearum 502 into soil led to the reduction of cucumber plants yield from 14.6 t/ha in the control sample to 8.8 t/ha in the AIB sample, with a yield loss of 40 %. The inoculation of cucumber seeds with T. viride 017 fungus secured an increase in yield (up to 13.5 t/ha) by 35 % compared with the AIB sample. It was established that pre-sowing treatment of seeds with T. viride 017 fungus contributed to a reduction in the unit cost of production by UAH 777 per hectare (24.2 %), an increase in conditional profit by UAH 36.654 per hectare.
